A
motherboard is the central
printed circuit board (PCB) in many modern
computers and holds many of the crucial components of the system, while providing connectors for other peripherals. The motherboard is sometimes alternatively known as the
main board,
system board, or, on
Apple computers, the
logic board.
It is also sometimes casually shortened to
mobo.
History
Prior to the advent of the
microprocessor, a computer was usually built in a card-cage case or
mainframe with components connected by a
backplane consisting of a set of slots themselves connected with wires; in very old designs the wires were discrete connections between card connector pins, but printed circuit boards soon became the standard practice. The
Central Processing Unit, memory and
peripherals were housed on individual printed circuit boards which plugged into the backplane.
During the late 1980s and 1990s, it became economical to move an increasing number of peripheral functions onto the motherboard (
see below). In the late 1980s, motherboards began to include single ICs (called
Super I/O chips) capable of supporting a set of low-speed peripherals:
keyboard,
mouse,
floppy disk drive,
serial ports, and parallel ports. As of the late 1990s, many personal computer motherboards supported a full range of audio, video, storage, and networking functions without the need for any
expansion cards at all; higher-end systems for
3D gaming and
computer graphics typically retained only the graphics card as a separate component.
The early pioneers of motherboard manufacturing were
Micronics,
Mylex,
AMI,
DTK,
Hauppauge,
Orchid Technology,
Elitegroup,
DFI, and a number of Taiwan-based manufacturers.
Popular personal computers such as the
Apple II and
IBM PC had published schematic diagrams and other documentation which permitted rapid
reverse-engineering and third-party replacement motherboards. Usually intended for building new computers compatible with the exemplars, many motherboards offered additional performance or other features and were used to upgrade the manufacturer's original equipment.
The term mainboard is archaically applied to devices with a single board and no additional expansions or capability. In modern terms this would include
embedded systems and controlling boards in televisions, washing machines, etc. A motherboard specifically refers to a printed circuit with the capability to add/extend its performance and capabilities with the addition of "daughterboards."
Overview
An
Acer E360 motherboard made by
Foxconn, from 2005, with a large number of integrated peripherals. This board's
nForce3 chipset lacks a traditional
northbridge.
Most computer motherboards produced today are designed for
IBM-compatible computers, which currently account for around 90% of global PC sales. A motherboard, like a
backplane, provides the electrical connections by which the other components of the system communicate, but unlike a backplane, it also hosts the central processing unit, other subsystems, and devices.
Motherboards are also used in many other electronics devices such as mobile phones, stop-watches, clocks, and other small electronic devices.
A typical
desktop computer has its
microprocessor,
main memory, and other essential components on the motherboard. Other components such as
external storage, controllers for
video display and
sound, and
peripheral devices may be attached to the motherboard as plug-in cards or via cables, although in modern computers it is increasingly common to integrate some of these peripherals into the motherboard itself.
An important component of a motherboard is the microprocessor's supporting
chipset, which provides the supporting interfaces between the CPU and the various buses and external components. This chipset determines, to an extent, the features and capabilities of the motherboard.
Modern motherboards include, at a minimum:
- slots into which the system's main memory is installed (typically in the form of DIMM modules containing DRAM chips)
- slots for expansion cards (these interface to the system via the buses supported by the chipset)
- power connectors, which receive electrical power from the computer power supply and distribute it to the CPU, chipset, main memory, and expansion cards.
The Octek Jaguar V motherboard from 1993. This board has 6
ISA slots but few onboard peripherals, as evidenced by the lack of external connectors.
Additionally, nearly all motherboards include logic and connectors to support commonly-used input devices, such as
PS/2 connectors for a
mouse and keyboard. Early
personal computers such as the
Apple II or
IBM PC included only this minimal peripheral support on the motherboard. Occasionally video interface hardware was also integrated into the motherboard; for example, on the Apple II and rarely on IBM-compatible computers such as the
IBM PC Jr. Additional peripherals such as
disk controllers and
serial ports were provided as expansion cards.
Given the high
thermal design power of high-speed computer CPUs and components, modern motherboards nearly always include
heat sinks and mounting points for
fans to dissipate excess heat.
CPU sockets
A CPU socket or slot is an electrical component that attaches to a printed circuit board (PCB) and is designed to house a CPU (also called a microprocessor). It is a special type of integrated circuit socket designed for very high pin counts. A CPU socket provides many functions, including a physical structure to support the CPU, support for a heat sink, facilitating replacement (as well as reducing cost), and most importantly, forming an electrical interface both with the CPU and the PCB. CPU sockets can most often be found in most desktop and
server computers (laptops typically use surface mount CPUs), particularly those based on the Intel
x86 architecture on the motherboard. A CPU socket type and motherboard chipset must support the CPU series and speed. Generally, with a newer AMD microprocessor, you need only select a motherboard that supports the CPU and not be concerned with the chipset.
Integrated peripherals
Block diagram of a modern motherboard, which supports many on-board peripheral functions as well as several expansion slots.
With the steadily declining costs and size of
integrated circuits, it is now possible to include support for many
peripherals on the motherboard. By combining many functions on one
PCB, the physical size and total cost of the system may be reduced; highly-integrated motherboards are thus especially popular in
small form factor and budget computers.
For example, the
ECS RS485M-M, a typical modern budget motherboard for computers based on
AMD processors, has on-board support for a very large range of peripherals:
- USB 2.0 controller supporting up to 12 USB ports
- IrDA controller for infrared data communication (e.g. with an IrDA-enabled cellular phone or printer)
- temperature, voltage, and fan-speed sensors that allow software to monitor the health of computer components
Expansion cards to support all of these functions would have cost hundreds of dollars even a decade ago; however, such highly-integrated motherboards are available for as little as $30 in the USA.
Peripheral card slots
A typical motherboard of 2009 will have a different number of connections depending on its standard.
A standard ATX motherboard will typically have 1x PCI-E 16x connection for a graphics card, 2x PCI slots for various expansion cards, and 1x PCI-E 1x (which will eventually supersede
PCI). A standard
EATX motherboard will have 1x PCI-E 16x connection for a graphics card, and a varying number of PCI and PCI-E 1x slots. It can sometimes also have a PCI-E 4x slot. (This varies between brands and models.)
Some motherboards have 2x PCI-E 16x slots, to allow more than 2 monitors without special hardware, or use a special graphics technology called SLI (for Nvidia) and Crossfire (for ATI). These allow 2 graphics cards to be linked together, to allow better performance in intensive graphical computing tasks, such as gaming and
video editing.
As of 2007, virtually all motherboards come with at least 4x
USB ports on the rear, with at least 2 connections on the board internally for wiring additional front ports that are built into the computer's case.
Ethernet is also included now. This is a standard networking cable for connecting the computer to a
network or a
modem. A sound chip is always included on the motherboard, to allow sound output without the need for any extra
components. This allows computers to be far more
multimedia-based than before. Cheaper machines now often have their graphics chip built into the motherboard rather than a separate card.
Temperature and reliability
Motherboards are generally
air cooled with
heat sinks often mounted on larger chips, such as the
Northbridge, in modern motherboards. If the motherboard is not cooled properly, it can cause the computer to
crash.
Passive cooling, or a single fan mounted on the
power supply, was sufficient for many desktop computer CPUs until the late 1990s; since then, most have required
CPU fans mounted on their
heat sinks, due to rising clock speeds and power consumption. Most motherboards have connectors for additional
case fans as well. Newer motherboards have integrated temperature sensors to detect motherboard and CPU temperatures, and controllable fan connectors which the
BIOS or
operating system can use to regulate fan speed. Some higher-powered computers (which typically have high-performance microprocessors, large amounts of
RAM, and high-performance
video cards) use a
water-cooling system instead of many fans.
Some
small form factor computers and
home theater PCs designed for quiet and energy-efficient operation boast fan-less designs. This typically requires the use of a low-power CPU, as well as careful layout of the motherboard and other
components to allow for heat sink placement.
A 2003 study found that some spurious computer crashes and general reliability issues, ranging from screen image distortions to
I/O read/write errors, can be attributed not to
software or peripheral
hardware but to aging
capacitors on PC motherboards. Ultimately this was shown to be the result of a faulty electrolyte formulation.
For more information on premature capacitor failure on PC motherboards, see capacitor plague. Motherboards use
electrolytic capacitors to filter the
DC power distributed around the board. These capacitors age at a temperature-dependent rate, as their water based
electrolytes slowly evaporate. This can lead to loss of capacitance and subsequent motherboard malfunctions due to
voltage instabilities. While most capacitors are rated for 2000 hours of operation at 105
°C, their expected design life roughly doubles for every 10
°C below this. At 45
°C a lifetime of 15 years can be expected. This appears reasonable for a computer motherboard. However, many manufacturers have delivered substandard capacitors, which significantly reduce life expectancy. Inadequate case cooling and elevated temperatures easily exacerbate this problem. It is possible, but tedious and time-consuming, to find and replace failed capacitors on PC motherboards; it is less expensive to buy a new motherboard than to pay for such a repair.
Form factor
microATX form factor motherboard
Motherboards are produced in a variety of sizes and shapes called
computer form factor, some of which are specific to individual
computer manufacturers. However, the motherboards used in IBM-compatible
commodity computers have been standardized to fit various
case sizes. , most
desktop computer motherboards use one of these standard form factors—even those found in
Macintosh and
Sun computers, which have not traditionally been built from commodity components. The current desktop PC form factor of choice is
ATX. A case's, motherboard's and PSU's form factor must all match, though some smaller form factor motherboards of the same family will fit larger cases. For example, an ATX case will usually accomodate a
microATX motherboard.
Laptop computers generally use highly integrated, miniaturized and customized motherboards. This is one of the reasons that laptop computers are difficult to upgrade and expensive to repair. Often the failure of one laptop component requires the replacement of the entire motherboard, which is usually more expensive than a desktop motherboard due to the large number of integrated components.
Nvidia SLI and ATI Crossfire
Nvidia SLI and
ATI Crossfire technology allows two or more of the same series
graphics cards to be linked together to allow faster graphics-processing capabilities. Almost all medium- to high-end Nvidia cards and most high-end ATI cards support the technology.
They both require compatible motherboards. There is an obvious need for 2x
PCI-E 16x slots to allow two cards to be inserted into the computer. The same function can be achieved in 650i motherboards by
NVIDIA, with a pair of x8 slots. Originally, tri-Crossfire was achieved at 8x speeds with two 16x slots and one 8x slot; albeit at a slower speed. ATI opened the technology up to
Intel in 2006, and all new Intel chipsets now support Crossfire.
SLI is a little more proprietary in its needs. It requires a motherboard with Nvidia's own
NForce chipset series to allow it to run (exception: select
Intel X58 and
P55 chipset-based motherboards).
It is important to note that SLI and Crossfire will not usually scale to 2x the performance of a single card when using a dual setup. They also do not double the effective amount of
VRAM or memory bandwidth.
Bootstrapping using the BIOS
Motherboards contain some
non-volatile memory to initialize the system and load an
operating system from some external peripheral device. Microcomputers such as the Apple II and IBM PC used
ROM chips, mounted in sockets on the motherboard. At power-up, the central processor would load its program counter with the address of the boot ROM and start executing ROM instructions, displaying system information on the screen and running memory checks, which would in turn start loading memory from an external or peripheral device (disk drive). If none is available, then the computer can perform tasks from other memory stores or display an error message, depending on the model and design of the computer and version of the BIOS.
Most modern motherboard designs use a
BIOS, stored in an
EEPROM chip soldered to the motherboard, to
bootstrap the motherboard. (Socketed BIOS chips are widely used, also.) By booting the motherboard, the memory, circuitry, and peripherals are tested and configured. This process is known as a computer
Power-On Self Test (POST) and may include testing some of the following devices:
- External USB memory storage device
Any of the above devices can be stored with
machine code instructions to load an
operating system or
program.
See also
